1. Field
Embodiments of the present invention are related to the field of modulators for video displays and, in particular, to micro-mechanical light modulator systems for visible light in projection and virtual projection systems.
2. Background
For video projection systems, there are currently two favored micro-display technologies that are growing in use, LCOS (Liquid Crystal on Silicon) and DMD (Digital Micro-mirror Display). Both technologies can be constructed primarily using silicon IC (Integrated Circuit) fabrication methodologies. Both technologies provide very small displays with very large numbers of pixels and have been successfully demonstrated to generate very large high definition displays. The displays can be rendered on front projection and rear projection screens as well as in virtual projectors which create the perception of a large image projected at a comfortable distance.
LCOS micro-displays are primarily reflective and operate by rotating the polarization of incident light. By illuminating the display with light of one polarization, and then filtering out that polarization from the projected light, an image is produced from the remaining unfiltered light. The requirements for strict polarization control add to the cost of LCOS based systems and reduce the brightness of the resulting projection system. Grayscale in LCOS panels is most commonly realized by varying the analog voltage in proportion to the desired gray level. Alternatively, some systems realize grayscale digitally by varying the proportion of time a pixel spends in the two polarization states. The latter method is referred to as duty cycle control or pulse width modulation control.
Transmissive and reflective LCDs (Liquid Crystal Display) are also commonly used in video projection systems. They can be made to either allow or block all light or to rotate the polarization of incident light. LCD displays require unique fabrication methodologies, have a slower reaction time, and cannot achieve the fill factor possible with displays built on a semiconductor backplane. While many projector systems use three display panels or CRTs, one for each of red, green, and blue, a projector system that uses a single panel sequentially for all three color components is smaller, lighter, and lower cost than a three-panel system. However, with video frame rates of 60 Hz, the slow reaction time of LCD technologies limits their usefulness in single-panel systems.
DMD micro-displays use a MEMS (Micro-Electro-Mechanical System) transducer in the form of a hinged micro-mirror. Light impinging on the micro-mirror is directed either into the collection field of a projection lens or to a light scavenger. As with some LCOS micro-displays, video grayscale is accomplished by varying the dwell time that light is directed into the projection lens collection field. The DMD micro-display does not rely on polarization effects to produce an image, so the losses and expenses of polarization-controlling systems are avoided.
In a DMD micro-display some portion of the surface is not covered by the mirror, these include the divots associated with the support pads for each mirror and areas between the mirrors. Incident light hitting these surfaces is partially absorbed and partially scattered, reducing brightness and contrast. In addition, any deviation in the angles of the mirrors also reduces the brightness of any off-angle pixel and reduces the brightness uniformity of the display panel.
The mechanical design of a DMD micro-display further requires a certain amount of inter-pixel spacing between the mirrors to support the electromechanical actuators and to accommodate the tilting of the mirrors. This spacing cannot be reduced easily, even when the size of the mirrors is reduced. As a result, if the micro-display is reduced in size beyond some level, the brightness and contrast of the display may be significantly reduced.